Prestige 110 Low flow in one zone

Discussion in 'Boiler Forum' started by T2, Nov 27, 2013.

  1. T2

    T2 In the Trades

    Portland, ME
    Hey All,
    I have a three unit apartment and last year I replaced my 25 year old boiler with a Prestige 110. During the overhaul I also replaced the base board with old cast iron radiators in two of the units. Recently I replaced the baseboard in the third unit with cast iron rad's. Absolutely love the rad's! The problem that I now have, is that one zone that has very low flow, and one is moderate but not what it was lat year. I believe there is air in the system and I have tried to purge it by isolating each zone, and flushing it, then bleeding the valves on the rads. I might add, that the first zone in the loop goes to the third floor and is still on baseboard, and I can not find a bleeder valve in the loop so I assume there is some air in there. Thing is, I never turn on the heat to third floor so it is pretty much isolated from the system. There is also flow in the problem zone, in each line, because I can get one line to respond by shutting off the others, it just drops when I start to open up the others.

    I have attached a picture of the system. The first zone goes to the third floor. The second zone with 4 lines goes to the second floor and has good flow. The third zone with five lines goes to my first floor and is the zone that I am having problems with. The fourth zone goes to an in-law unit off the back house which is the zone I just revamped.

    So the questions I have are several. How do I properly bleed this new system? When I did it yesterday, I had all the thermostats turned on and started out back and then purged the 1st, 2nd, and third floors. Then bleed them. While doing this the water pressure did drop and the boiler shut down do to pressure, which I got back by opening the regulator valve a touch, but I don't believe my procedure was entirely correct. The pressure on the boiler is now about 12 psi.
    2nd, Do I have to put a bleeder valve on the third floor, or can it wait for the final overhaul?

    Lastly, What is the best temp to run my rad's at for efficiency? Yes, that is loaded question. Last year I ran them at 120 and the place was the warmest it's ever been. I over sized on some the rad's. When we hit a cold snap I bumped it up to 150. Now, I know that is far to low for baseboard, hence the overhaul. The unit out back has two zones, one of which had two rads on it last year, the other had the baseboard in the loop. (long story)
    That's where I am at. Any feedback would be gratefully appreciated.

    Attached Files:

  2. Dana

    Dana In the trades

    I don't see the pump(s) in that plumbing sculpture. What do you have, and how are they configured?

    Hanging the expansion tank sideways is asking for a catastrophic leak when the vibration on the weight of it's internal water mass eventually breaks the pipe. It's far safer/better to hang it vertically, tank below the plumbing, but if there isn't sufficient clearnace, vertically with the tank above the plumbing works.

    Operating it at the lowest possible temperature than actually heats the place is what provides the most condensing efficiency. The domninant factor in condensing is the temperature of the water entering the boiler, not the output temp, but at a typical 20F delta-T you won't be getting ANY condensing at the boiler with 150F output. Condensing begins at 122-126F (depending on the boiler and how well it's tuned), at which point it'll be running ~89-90% combustion efficiency. To get in to the mid-90s the return water has to be in the 110-115F range or lower. Using the outdoor reset function and letting the boiler auto-adjust the output temp up/down with changing output temps can automate it to max-efficiency, if you take the time to set up the outdoor reset curve properly, and tweak it in.

    On a 3 story house with the boiler in the basement 35' below the top of the highest radiator, 12psi is on the low side. When cold you'll get about 0.43 psi per foot of elevation, so if the top of the system is more than (12psi/0.43= ) 28 feet above where you're measuring the pressure, water only gets to the top of the system when pumping, and purging air at the top becomes nearly impossible. With most low mass boilers even on a 1-story building it's often better to run it at ~15psi to keep it from sizzling under modest-flow conditions, even with the pumping configuration is pumping toward, not away from the boiler. If it's pumping away from the boiler you may have to raise it to the 18-20 psi range to keep the micro-boil on the heat exchanger from audibly sizzling under some conditions.

    Air will tend to migrate to the highest elevation of the system. Once you have sufficient system pressure that the water actually makes it to the top without actively pumping, you can bleed it from there. A riser teed off a fatter (=slower flow) section of pipe, with a vent at the top located an inch higher than the tallest radiator would eventually purge the air if there is at least some flow. A bleeder valve at every radiator is sometimes useful if the system is prone to vapor-locking with a bubble in some other branch at some other elevation on the system.

    You'll need to pre-charge the expansion tank to the new improved system pressure, and size the expansion tank to the now higher system volume when swapping out baseboard for bulky radiators.
    Last edited: Nov 27, 2013

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